This invention relates in general to sensors and in particular to a high frequency apparatus and method for sensing the accumulation of particles having an electrical loss component such as soot in diesel engine oil.
During usage of a diesel engine, for example, the crankcase oil gradually experiences particle accumulation in the form of soot, which is a combustion by-product, in the combustion chamber of the engine. Portions of soot may then be transferred in small amounts to the crankcase oil. When the soot builds up to an unacceptable amount, such as a predetermined threshold percentage, the lubricating quality of the oil is diminished. When this occurs it may be necessary to change the crankcase oil whenever the soot content reaches an unacceptable value. For this purpose, it is desirable to measure the soot content percentage in the crankcase oil in order to detect the presence of the unacceptable percentage of soot.
Further, diesel engine oil is degraded by the presence of soot particles that increasingly accumulate with engine use. In addition, diesel oil is degraded as its additives are depleted and oil acidity is increased. It is becoming increasingly important for the efficient maintenance of diesel engines to have an on-board sensor that will accurately detect soot concentrations.
Microwave probes are known to be used for detecting soot in diesel engine oil such as the one disclosed in U.S. Pat. No. 4,345,202 issued to Nagy et al. Nagy discloses a microwave probe used to detect soot up to concentration levels of about five percent. This sensor used a single microwave frequency to characterize the real part of the relative permittivity for used diesel oil. From this data it was determined that the microwave probe could be used to measure the real part of the permittivity and thus determine soot content of up to about five percent. However, the more advanced diesel engines in today""s markets frequently require detecting soot concentrations at levels up to about eight percent. This new requirement for detecting higher soot concentrations presents problems for existing electromagnetic sensors, which have not been able to accurately measure the soot content in diesel engine oils when soot content is greater than about four percent. Above a four percent soot concentration level, various small soot particles appear to agglomerate to form large electrically lossy particles. These agglomerate particles adversely affect the ability of these sensors to accurately measure soot concentrations because of the change in the particles geometry and increase in electrical losses.
It is known that even though soot particles are conductors rather than dielectric they can increase the relative permittivity or relative dielectric constant of a dielectric fluid, such as diesel oil for example, because very small conductive particles in a dielectric form what is commonly called an artificial dielectric. Thus, the proposition that the soot content of engine oil can be measured by measuring the relative permittivity of the oil is viable provided there are no other factors affecting the relative permittivity of the oil. The buildup of non-soot contaminants during engine service is one factor that can influence the relative permittivity. In addition, the formulation of the oil itself is a contributor to variable relative permittivity. That is, various engine oils have different compositions. There are synthetic, as well as natural petroleum bases, and there are various types of additives used by the numerous oil manufacturers. As long as these variables tend to influence the relative permittivity to a significant extent, then that relative permittivity parameter cannot be used as a measurement of soot content. That is, non-soot constituents of engine oil can contribute enough to relative permittivity measurements to make those measurements unsuitable for a measure of soot content.
Thus, it would be advantageous to provide an apparatus and method for accurately detecting the accumulation of particles having an electrical loss component, such as the soot content in diesel engine oil, at levels of up to about eight percent.
An apparatus and method is provided for detecting soot content in diesel engine oil, especially at concentrations greater than about four percent. This may prevent oil from being changed prematurely, which may provide a significant economic gain to an operator of a fleet of vehicles, for example. Exemplary embodiments of the present invention allow for determining high-level soot content (i.e., up to and greater than four percent) based on the relative permittivity of the oil without the effects of non-soot contaminants adversely impacting that determination.
One exemplary embodiment of a method for detecting soot content in oil may comprise generating a first waveform at a first frequency and a first voltage level associated with this first frequency to produce a standing wave with a voltage null point at a first detector location with the null point having a first null voltage level. At least a portion of the standing wave will be passed within a portion of the oil having little to no soot content (xe2x80x9ccleanxe2x80x9d oil). The first voltage level, associated with this first frequency, and first null voltage level may be used as a baseline for determining the soot content. A second waveform may be generated for an oil sample under investigation for soot content at a second frequency and a second voltage level associated with this second frequency to place a signal associated with the voltage null point at the first detector location with this null point having a second null voltage level. The soot content of the sampled oil may be determined by calculating the ratio of the difference between the second voltage level and the first voltage level and dividing this difference by the second null voltage level. The difference in the first and second voltage levels for generating the waveforms is associated with the magnitude of the diesel oil""s dielectric characteristics while the difference in the null voltage levels is associated with the imaginary part (loss part) of the oil""s dielectric characteristics. This ratio represents the normalization of the magnitude of the oil""s dielectric characteristics to the losses associated with the soot concentration.
One exemplary embodiment of an apparatus for detecting soot content in oil may comprise a variable frequency microwave source coupled with a probe that is immersed within the oil. A microwave detector may be coupled with the probe to detect a signal level of a null voltage proximate the microwave detector. A microcontroller such as a processor may be provided that is coupled with the microwave source, the probe and the microwave detector. The processor may be configured to calculate a ratio of a difference between a first or baseline voltage level associated with a first standing wave and a second voltage level associated with a second standing wave, over the second null voltage level. The second voltage level and the second null voltage level may be determined in response to a change in a relative permittivity of the diesel oil and wherein the ratio is indicative of the percentage of soot content.